To fabricate a hollow member having regions differing in wall thickness from each other, it has been a past practice to combine another member separate from the hollow member with the hollow member as shown in, for example, Patent Document 1. As shown in FIG. 13, in the case of a hollow member 80 disclosed in this Patent Document, a reinforcement member 82 is fitted onto a hollow base body 81 in one piece, cut out of an aluminum extruded section, thereby making up a central part of the hollow member 80 so as to be lager in wall thickness. Further, the hollow member 80 is for use as a guard bar for a vehicle. Patent Document 1: JP-UM-A 07(1995)-023617
For fabrication of the hollow member 80 having parts varying in wall thickness, a separate member is fitted thereto as disclosed in the past, so that an increase in the number of man-hours for production is unavoidable. In addition, in the case of this hollow member 80, variation is prone to occur to location of the reinforcement member 82 when the reinforcement member 82 is fitted, so that there are limitations to stability in quality of the hollow member 80.
Further, since the conventional hollow member 80 is made up such that separate members are fitted thereto, it is difficult to fabricate one gradually varying in thickness. Assuming the case of fabricating a hollow member gradually varying in thickness, a secondary work such as cutting, and so forth will be required after the separate members are combined with the hollow member. Accordingly, it inevitably becomes complex to fabricate a hollow member of such a makeup as described.